Generally, a film of a metal or a metal compound, such as W (tungsten), WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride)or TiSi (titanium silicide), is deposited to form a wiring pattern on a semiconductor wafer, to fill up holes between wiring lines or to form wiring patterns and fill up holes between wiring lines in a semiconductor integrated circuit fabricating process.
Methods of forming such metal thin films are classified into those of H.sub.2 reduction system (hydrogen reduction system), those of SiH.sub.4 reduction system (Silane reduction system) and those of SiH.sub.2 Cl.sub.2 reduction system (dichlorosilane reduction system). When forming a wiring pattern by the method of SiH.sub.2 Cl.sub.2 reduction system, dichlorosilane gas is used as a reducing gas and a W or a WSi film (tungsten silicide film) is formed at a high temperature on the order of 600.degree. C. When forming a wiring pattern by the method of SiH.sub.4 reduction system, silane gas is used as a reducing gas and a W or WSi film is formed at a low temperature on the order of 350.degree. C.
The method of H.sub.2 reduction system is applied mainly to filling up holes in the surface of a wafer, such as holes between wiring lines, uses hydrogen gas as a reducing gas and deposits a W film at a temperature in the range of about 400.degree. to about 430.degree. C.
All those methods use, for example, WF.sub.6 (tungsten hexaf luoride). A conventional tungsten film forming method will be explained. A thin Ti/TiN film, for instance, is formed as a barrier metal layer on a surface of a semiconductor wafer before forming a tungsten film. Film forming gases including WF.sub.6 gas, SiH.sub.4 gas, H.sub.2 gas, Ar gas, N.sub.2 gas and the like are supplied into a film forming chamber to deposit tungsten seed crystal grains on the surface of the barrier metal layer.
The film forming chamber is evacuated temporarily to a base pressure to remove residual gases from the film forming chamber, and then the Ar gas, H.sub.2 gas and N.sub.2 gas are supplied into the film forming chamber to set the film forming chamber quickly at a process pressure. Subsequently, WF.sub.6 gas is supplied at a predetermined flow rate into the film forming chamber to deposit a tungsten film by hydrogen reduction using H.sub.2 gas without using SiH.sub.4 gas. Thus, for example, filling up holes and forming a wiring layer are carried out simultaneously.
The development of multilayer semiconductor integrated circuits, and the progressive miniaturization and rise in the level of integration require further reduction of width of lines and diameters of holes. When a wiring pattern is miniaturized, the resistance of wiring lines increases accordingly. Resistivity of wiring lines low enough for conventional design must be reduced further when wiring patterns are miniaturized.
However, it has been difficult to form tungsten films having a satisfactorily low resistivity and meeting new design by the foregoing conventional tungsten film forming method.
A method intended to form a tungsten film having a reduced resistivity to solve the foregoing problems supplies a bo-ron-containing gas such as diborane (B.sub.2 H.sub.6) gas with Ar gas and N.sub.2 gas into the film forming chamber to form a tungsten film of tungsten crystal grains of large grain sizes to reduce resistivity. This method, however, is incapable of forming the tungsten film having satisfactorily low resistivity, and the nitrogen-diluted borane gas produces a solid by polymerization in gas supply pipes and the solid clogs the gas supply pipes.
Generally, unnecessary films are deposited in the processing vessel of a film forming system as a film forming process is repeated certain cycles, and the films fall off in particles. Therefore, a cleaning process is carried out at regular or irregular intervals to remove the unnecessary films by supplying a cleaning gas, such as ClF.sub.3 gas, into the processing vessel. The cleaning gas remains, though in only a very small amount, in the processing vessel after cleaning and Cl and F atoms contained in ClF.sub.3 gas are introduced into the surface of a semiconductor wafer and act as detrimental impurities.
The present invention has been made in view of such problems and has been created to solve those problems effectively. It is therefore an object of the present invention to provide a tungsten film forming method capable of forming a tungsten film having a low resistivity.